1. Field of the Invention
The present invention relates generally to a write clock generator incorporated in a time base corrector for a video tape recorder and more specifically to a write clock generator which can generate a write clock signal having accurate phase information and accurate frequency information on video signals reproduced from a video tape.
2. Description of the Prior Art
In a time base corrector for a video tape recorder, a synchronizing signal SYNC and a burst signal BURST are separated from a video signal VDIN reproduced from a tape. The write clock generator generates a write clock signal WCK and a write start pulse signal WZERO involving time base fluctuations (i.e. jitter) in synchronism with the above separated signals SYNC and BURST. The write start pulse signal WZERO serves as a head address designation signal for each scanning line to a memory. Memory addresses are incremented in sequence in response to the write clock signal WCK in order to write the reproduced video signal VDIN in a memory scanning line by scanning line. The video signal stored once in the memory is read in response to a read start pulse RZERO and a read clock signal RCK generated from a read clock generator in response to a stable reference pulse signal VDREF. The read video signal is outputted from the time base corrector as a reproduced video signal VDOUT without including jitter together with a synchronizing signal SYNCX, a burst signal BURSTX and a blank pulse BLKX all applied from the read clock generator.
In the time base corrector as described above, in order to accurately write the reproduced video signal in the memory, it is necessary to generate a write clock signal WCK the phase of which accurately varies according to jitter included in the reproduced video signal VDIN, because color offset will be produced.
Japanese Patent App. No. 60-24669 discloses a write clock generator which can accurately generate a write clock signal WCK including phase information and frequency fluctuation information on the reproduced video signal VDIN. In this method, the write clock signal WCK is generated in phase-synchronism with the reproduced burst signal BURST. Although the initial phase information can be included, since it is impossible to give frequency fluctuation information to the write clock signal WCK, this method is not practical in the case where the reproduced video signal includes relatively great frequency fluctuations.
To overcome the above problem, there has been proposed another method in which a write clock signal WCK is generated on the basis of frequency fluctuation information obtained from the reproduced horizontal synchronizing signal SYNC and phase information obtained from the reproduced burst signal BURST. In the write clock generator of this method, the reproduced horizontal synchronizing signal SYNC is applied to a phase locked loop circuit to form a synchronizing signal including frequency fluctuations involved in the reproduced video signal VDIN, and the formed synchronizing signal is phase-shifted so as to synchronize with the phase of the reproduced burst signal BURST. In this method, however, there exist such shortcomings that the circuit is complicated, because the frequency fluctuations are suppressed within .+-.half wave in digital fashion and further the synchronizing circuit is configured in analog fashion.